A highlighter is a special pen with a wide, pale, bright tip — often yellow, pink, green, or orange — that you pull across words on a page so they jump out when you look back. Students and office workers use it to mark lines they want to find again. The colour is light on purpose: you can still read the print underneath. It is highlight + -er, “the thing that highlights.” You highlight with it; the marks are highlights on the page. Though actually, people also highlight text on a phone with a finger, and that is the same idea with no pen at all. This word is the pen. It is a school-bag and desk word, not a formal one.